Suzuki: Emissions failed, suzuki sidekick, oxygen sensor

Question
Why would my 1993 Suzuki sidekick fail emissions? He said it was the Nox part.

Answer
I haven't done emissions since 89, and am now doing fleet diesels, so my knowledge of this is out of date and limited.  In addition, Colorado, my state of residence, being a high altitude state, does not have nox issues.
the following could be at fault:
Timing too advanced, EGR valve inoperative or nearly so, Catalytic converter, oxygen sensor.
scotty